Day 4 of Software Architecture 2012 & Agile and OO design

Today was the last day of Software Architect 2012. Last day was dedicated to workshops and I decided to join Allen Holub session: “Agile/OO design from start to finish”. After these days I become a fan of A. Holub and I hope that I will be able to implement a correct OO design. It is pretty challenging, but I promise that I will try to write about this in future.

Here are some notes from the presentation that I considered important. A part of them are well known by all of you:
Agile is based on communication, but there are people that will not be able to integrate in a team environment. Even if they are great on technical side and have a lot of knowledge, they are not able to communicate and to share data. Sharing is one of the most important things in Agile. The biggest enemy for Agile is EGO. Because of this people will not be able to communicate and different roles will appear – “I am the MASTER and the team need to do as I say.” – WRONG.
When we are talking about user-stories, management should understand the concept of points and the relations between points and duration – there is no relationship. Also when the team say, there are 50% percent chances to not be able to finish at time, than the chances are pretty high.
If you have a bunch of user-stories, start with the one that are the most risky. You will need time to clarify and resolve them. Also if you have a complicate or a user story that is very big than you should slit him? One good solution here is for the first sprint to consider only one happy flow, after that another, or an exceptional flow and so on. In this way we will be able to make it more and more complex at each iteration. Your scope is to have something that is fully functional at the end of every iteration.
Be strong and in the moment when someone wants to take off a man from the team say “NO!” This action can affect the project velocity; because of this there are chances not to finish all the user stories at the end of the iteration.
I didn’t use the SCRUM and SPRINT terminology.  Why? SCRUM is not the only way to be AGILE. From some perspective SCRUM is not agile, in the current “implementation”.
I will finish this post blogs about this conference with two cited that I heard today and I like:
“A ‘bug’ is not a logic error; it is a missing unit test.”
“Each team member is responsible for his own word and the team is responsible for the entire project (all the code and artifacts that are generated).”

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Day 3 of Software Architecture 2012 & Basic Principles of an Object

Day 3 of Software Architect 2012 from London has passed. Today was a great day for science also. I went at 3 sessions sustained by Allen Holub. This man is not only a great speaker, but also knows the programing art extremely good.

Like in the other post about conferences where I participate, I will present in some words the things that I consider important.

Now we are living in a world that where OOP is everywhere. We are trying to create a model for each object from this world, but there are times when we forget base things. An object should expose a set of functionalities and not information. Sounds odd in our days, where everything is a POCO or a service, but this is how we should write our classes.

A world that is formed from POCO and services is not more than a procedural programing. In this case let’s change the name from OO to Procedural Programing. An object should be defined by what can do, what actions contain – public methods; and not by data that he contains. The content is not important; the functions that are exposed are the most important.

This is easy to say, but trust me, it is very hard to implement. For example when we have a class that want to calculate the price with VAD and deduction of a product we create a separate class for this. This calculator class will access the price property of our product. But we don’t need something like this. It is not wrong to have a method that returns the final price of the product in our product class. Yes, of course, as input parameter or in constructor we can specify an instance of an object that knows how to calculate (the final formula).

 There are lot more thing to talk about this subject. I promise that every week I will have a dedicated post related to OOP and design. Until than I will buy this book and start reading.

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How to NOT expose a read only collection in C#

These days I had the opportunity to make a review over an ecommerce application. They tried to use CQRS and they almost succeeded. 
I notified a problem on their queries classes that can become a big problem in time, if you want to sell this solution as a platform. Also, from some perspective, these problems also violate the CQRS principle.
Let’s see some code now:

public class Order
{
    Collection<OrderItem> _items;   

    public IEnumerable<OrderItem> Items
    {
        get
        {
            return _items;
        }
    }
    ...
}

What do you see here strange?  We want to expose the OrderItem collection as a read only collection. To do this, we convert it to IEnumerable.
Hmmm… sounds good? Nope. Big mistake! Nobody stop us to convert the Items back to an ICollection and Add/Remove items from it.

Order order = new Order();
…
var orderItems = (Collection<OrderItem>)oder.Items;

What should we do? .NET framework has specials collections that can be used in these situations. I this case we should use IReadOnlyCollection<T>. This class will let the user to access items but he will not be able to modify the list (add or remove items from the collection).
To obtain the list as a read only collection we will need to use the AsReadOnly() method:

public class Order
{
    Collection<OrderItem> _items;   

    public IReadOnlyCollection<OrderItem> Items
    {
        get
        {
            return _items. AsReadOnly();
        }
    }
   
    ...
}

Under the hood, this method will create a new collection that point to our original items.
When expose items as “read-only”, you should always double check. There are many methods that permit us to cast items.

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Day 2 of Software Architecture 2012 & How an architect should be

This week I attended to Software Architecture 2012 conference from London. This was the second day of conference with 4 sessions per day on six tracks simultaneous. Here is the blog post of day one.
There were great sessions about how software design and architecture. I really enjoyed the keynote, where Simon Brown talked about how software architecture should be. One thing that he mentioned and I thing that is very important is what an architecture should do. I think that we know a lot of architecture that don’t write code anymore and don’t learn new technologies. They climb on the Ivory Tower and when we have a question throw a response like: “Implementation detail”. How you can define a solution when you don’t know the technology? Yep, that type of “guru” is only a PowerPoint architected and nothing more.

He is the perfect person that can draw components on a white paper and the first person that will run when a project will have problems. In that moment he will show the SAD (Software Architecture Documentation) but without giving you a real solution.
A pretty nice solution here (because you cannot be master of all technologies) is to let your ego at home and talk with technologies specialists. Design a good solution means making a team with this guys and with the rest of the developing team. They know the real problems that technologies have and what is the real limitation of a framework. On paper everything looks good.
As software architecture you should be able to share your knowledge, to couch and be mentor for other peoples. If you share your knowledge no one will come and steal your position. In reality, your value will increase because the team level will be higher and they will like working with you.
Don’t let people learn from their own experience. Try to share your experience with others, because we don’t want to reinvent the wheel again and again.

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Different methods to implement Message Aggregator pattern using Service Bus Topic – CorrelationId

In one of my last post I presented the Aggregator Pattern. The main purpose of this pattern is the ability of the consumer to combine (aggregate) messages. This pattern can be implemented in Windows Azure using Windows Azure Service Bus Queues or Topics.
There are two different implementation for this pattern. The implementations are extremely different and also can affect our performance.
The first implementation requires using the Session support from BrokeredMessage. What this means from the code perspective? We need to set the Session each time when we want to send a message. The consumer will start to consume the messages with the specific session id. This solution is simple and it works great when we don’t have a lot of messages. For example if we have only 9-10 messages.
The first advantage of this implementation is on the consumer side. We don’t need to create the consume before messages are added to the Service Bus. The messages will be persisted in the Service Bus infrastructure. One of the downside of this implementation is the numbers of the consumers that we can have for the same session. We can have only one consumer. Because of this, if we want to broadcast a message to more than one consumer … we will have a problem. Also, the current implementation of session doesn’t use any kind of hashing for the id field. Because of this, if we want to process thousands of messages per second, maybe we will have a problem.
Producer

Stream messageStream = message.GetBody<Stream>();
for (int offset = 0;
    offset < length;
    offset += 255)
        {                     
            long messageLength = (length - offset) > 255
                ? 255
        : length - offset;
            byte[] currentMessageContent = new byte[messageLength];
            int result = messageStream.Read(currentMessageContent, 0, (int)messageLength);
            BrokeredMessage currentMessage = new BrokeredMessage(
        new MemoryStream(currentMessageContent),
        true);
            subMessage.SessionId = currentContentId;
            qc.Send(currentMessage);
        }

Consumer

MemoryStream finalStream = new MemoryStream();
MessageSession messageSession = qc.AcceptMessageSession(mySessionId);
while(true)
{
    BrokeredMessage message = messageSession.Receive(TimeSpan.FromSeconds(20));
    if(message != null)
    {
                message.GetBody<Stream>().CopyTo(finalStream);
                message.Complete();
                continue;
    }
   break;
}

In the above example we are spitting a stream in small parts and sending it on the Service Bus.
But, what should we do if we want more than this. For example if we want to send messages to more than one consumer. In this case, if we need a way to group messages, like session we will need to think twice. We have some options for this situation. We can use the CorrelationId for this purpose of to add a custom property to the BrokeredMessage. CorrelationId use hashing, because of this is faster than the first option.
Both solutions will work, but we will encounter the same problem in both of them. How we can create a subscription for the given CorrelationId of property before starting receiving messages. This is the biggest problem that we need to resolve.
Before talking about this problem, I would like to talk a little about CorrelationId. This fields that can be set for each BrokeredMessage that is send on the wire. The advantage using is how we can define the filter. We have a pre-define filter for the CorrelationId that can be use when we want to create a subscription. Also each id is hashed; because of this the check is not made using string comparison.
But what is our problem using CorrelationId. We can broadcast a message to more than one subscriber, but… Yes, there is a but. We need to know the correlation id in the moment when we create the subscription. Why? Because the correlation id need to specify in the moment when we are creating the subscriber. Correlation id need to be specified for a subscriber as a CorrelationFilterExpression.
This is not the end of the road. We can find solutions for this problem. The trick is to notify the consumers before adding messages to the Service Bus Topic about the new group of messages that will be added to the system. For this purpose we can use the same Service Bus Topic, or another topic for this. We can have some special messages that have some custom property that describe what will be content of the next flow of messages with the given correlation id. Based on this information, each consumer will be able to decide if we want to receive the given messages.
The trick here is to register the subscribers before the moment when you start broadcast the messages with a given correlation id. The messages will be sending only to the subscribers that are already listening. Because of this, if you are not register from the beginning, there are chances to lose some of the messages.
You need some kind of callback or a timer. Because in a Service Bus pattern, the producer doesn’t know the numbers of subscribers, we cannot create a mechanism where each subscriber notifies the producer using another topic. We could have a waiting time (5s) on the producer side, after he sends the message that notify about the new correlation id.

Here is the code that we should have on the producer and on the consumer side.
-on the producer side, we need to create and send the message that contains the new correlation id. After this we will need to wait a period of time, until the consumers will be able to register to it.

BrokeredMessage message = new BrokeredMessage();
message.Properties["NewCorrelationId"] = 1;
message.Properties["Content"] = "new available cars for rent";
topicClient.Send(message);
Thread.Sleep(10000);

-creating the subscription that check if the message contains the property that specify the correlation id.

namespaceManager.CreateSubscription(
     “myTopic”,
     “listenToNewCorrelationIds”,
     new SqlFilterExpression("EXISTS(NewCorrelationId”));

-create the consumer that processes the message, by creating a new subscription.

SubscriptionClient client =
    SubscriptionClient.CreateFromConnectionString(
        connectionString,
        "myTopic",
        “listenToNewCorrelationIds”);
BrokeredMessage correlationIdMessage = client.Receive();
namespaceManager.CreateSubscription(
     “myTopic”,
     “listenToMyCorrelationId”,
     new CorrelationFilterExpression(correlationIdMessage.Properties["NewCorrelationId"]));
SubscriptionClient client =
    SubscriptionClient.CreateFromConnectionString(
        connectionString,
        "myTopic",
        “listenToMyCorrelationId);

... client.Receive() …

From performance perspective is would be better to use a different topic to send the messages that contains the new correlation id.
We can imagine a lot of implementation. What we need to remember when using correlation id that we need to create a subscription for the given correlation id before starting sending the messages to it. This waiting period that I described in the above paragraphs is the most challenging part. 
In conclusion, we should use session when we need to have only one consumer. If we need more than one consumer, than we should use correlation id.

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Day 1 of Software Architecture 2012 & Unit Test Patterns

This week I attended to Software Architecture 2012 conference from London. This was the first day of the conference and I decided to participate to a full day workshop about design patterns that was held by Andrew Clymer and Richard Blewett.
After this day I made a clearer image in my mind about some design pattern and how we can use it. There are a lot of new things that I discovered today. A part of them will be covered in future blog posts.
I looked over my today notes and I’m trying to extract some information for this blog post, but there are a lot of great things, that I feel the need to dedicate an entire post about it.
I will make only a small introduction into Unit Test Patterns. Any programmer should write unit tests. As we study design patterns in general we should learn also about unit test patterns.
In the last period of time the software industry become mature from many perspectives. The testing area, especially unit testing has reached his maturity. With this maturity, some pattern for unit testing appeared and guys don’t thing if you are a developer you should not know about this. This things are not for testers, are for you, for developers. Developers write unit test and not testers. Also understanding unit testing and pattern that are related to unit testing will improve our production code.
The simplest pattern is “Simple-Test”. This is the most basic unit test pattern that validate that for a valid input the expected outcome is obtained. In the case if our code contains an error trap, than we need to create a test for this case.
Suppose that we have a class Calculate that calculate the sum of two numbers. We will need to write a test that validate that the sum for two numbers is calculated correctly. This test will not guarantee that our code is valid and will work for any kind of input data. It only validate the most simple and basic happy flow.

public class Calculator
{
        public int Sum(int a, int b)
        {
            if (a == 0)
            {
                return b;
            }
            if (b == 0)
            {
                return a;
            }

            return a + b;
        }
}

[TestClass]
public class CalculatorTests
{
    Calculator calculator=new Calculator();

    [TestMethod]
    public void SumOfTwoSimpleNumbersReturnsTheSumOfNumbers()
    {
        Assert.AreEqual(10,calculator.Sum(2,8));
    }
}

Another well know pattern is “Code Path”. In the first pattern we didn’t look over the code. We tested it with the happy flow. The Code Path Pattern is used to test all the paths from our code. In this way, we will be more confident about out code quality. When we are writing this kind of unit tests we don’t look over the requirements, because we could miss some paths from our code. Code Path Pattern requires looking through the code in a “white box” method and covering all the possible paths. When we are using this pattern over legacy code you may be shocked about the number of lines of codes that are not used anymore.

[TestClass]
public class CalculatorTests
{
    Calculator calculator=new Calculator();

    [TestMethod]
    public void SumOfTwoSimpleNumbersReturnsTheSumOfNumbers()
    {
        Assert.AreEqual(10,calculator.Sum(2,8));
    }

    [TestMethod]
    public void SumWhenFirstNumberIsZeroReturnsSecondNumber()
    {
        Assert.AreEqual(5,calculator.Sum(0,5));
    }

    [TestMethod]
    public void SumWhenSecondNumberIsZeroReturnsFirstNumber()
    {
        Assert.AreEqual(3, calculator.Sum(3, 0));
    }

}

The last pattern that I will describe now is “Parameter Range”. When we are writing unit test, we should test our code with more than one input data. Even if we tested all the paths of our code this will not guarantee that our code is perfect.
For example in the previous example we should test what is happening if we calculate the sum of two big numbers that will generate a value that exceed the maximum value.

    [TestMethod]
    public void SumOfTwoSmallNumbersReturnsTheSumOfNumbers()
    {
        Assert.AreEqual(7, calculator.Sum(3, 4));
    }

    [TestMethod]
    public void SumOfTwoBigNumbersReturnsTheSumOfNumbers()
    {
        Assert.AreEqual(180, calculator.Sum(100, 80));
    }

    [TestMethod]
    public void SumOfTwoPrimeNumbersReturnsTheSumOfNumbers()
    {
        Assert.AreEqual(24, calculator.Sum(11, 13));
    }

I would like to congratulate the speakers of this workshop. I think that you made us thinking and see the design patterns from other perspectives.

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Windows Azure Service Bus Patterns - a comprehensive look at patterns that can be used in combination with Windows Azure Service Bus

In the last period of time I posted a lot about pattern that can be used in combination with Windows Azure Service Bus. This is the list of all the patterns that I presented until now:

1. Message Splitter Pattern
2. Message Filter Pattern 
3. Message Aggregator Pattern
4. Recipient List Pattern  
5. Resequencer Pattern
6. Content-Based Router Pattern
7. Scatter-Gather Pattern 
8. Dynamic Router Pattern 

If you want to find more information about Windows Azure Service Bus Topic, please follow this LINK.
For more information about Windows Azure Serbice Bus Queue please follow this LINK.

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